This invention relates to a system for transferring heat from a hot reservoir to a user by a thermal exchange fluid and having the particular freezer of extending lifetime of the thermal exchange fluid which is thermally degradable.
The invention has particular application to a system of domestic appliances wherein a single liquid phase thermal exchange fluid exchanges heat between the appliances and both a hot reservoir and a cold reservoir. The appliance system combines heating and cooling capability in simple appliance units and accumulates energy at off-peak hours and at low power levels for subsequent rapid release during peak use periods. These and other characteristics of the appliance system are described in more detail in the cited related applications and in the following patents. U.S. Pat. No. 3,888,303 describes a system of houseware units which are connectable to a source of thermal exchange fluid. U.S. Pat. No. 4,024,904 describes a range which exchanges heat between a pot or pan surface and a thermal exchange fluid in a heat exchanger by forced air convection.
Representative conditions to be satisfied by a thermal exchange fluid include a temperature range of -20.degree. F. to 575.degree. F. and a lifetime of more than 25 years without skilled preventive maintenance. The users operate intermittently with infrequent excursions to maximum working temperatures. Several commercial thermal exchange fluid types having generally satisfactory heat transfer characteristics over the cited temperature range have been designed for thermal stability, but even the most stable organic compounds change chemically at high temperatures over long periods. Undesirable effects which result from oxidation, cracking, and formation of higher polymers at high temperatures include vapor loss, viscosity increase and geling, and formation of flow and heat impeding deposits.
Thermal degradation over long periods is estimated conventionally from rates of undesirable effects occuring at higher temperatures over shorter periods. The scaling relation is an Arrehenius equation, D=Ae.sup. E/RT, where the constants A and activation energy E are determined from a range of degradation rates D at temperatures T. Once the constants are determined, degradation rates at lower temperatures are calculated. A representative accelerated test of a thermal exchange fluid is based on measurements over a period of several weeks of degradation products formed at temperatures ranging from about 600.degree. F. to 700.degree. F. At 650.degree. F., the more stable thermal exchange fluids have a degradation rate of approximately 1% per week. Most thermal exchange fluids have an activation energy such that their degradation rate is doubled for every 18.degree. F. increase in temperature. Accordingly, the following annual degradation rates are expected for lower temperatures: 600.degree. F.--6.5%/yr; 550.degree. F.--0.68%/yr. 500.degree. F.--0.056%/yr; 450.degree. F.--0.0036%/yr. At 450.degree. F., the degradation over 25 years is less than 0.1% so that continuous long term operation at such lower temperatures would be satisfactory. For cooking appliances, however, the higher temperatures are occasionally required within the appliance and are useful for compensating for thermal impedances in the system to attain desired high temperatures rapidly. A preferred maximum temperature of the thermal exchange fluid would be between 600.degree. F. and 550.degree. F.